Outline

Introduction: Changes in cerebral activities have occurred during transcutaneous vagus nerve stimulation with deactivation patterns in limbic and temporal brain structures. (Kraus et al.) These findings are similar to previous invasive (i)-VNS studies. (Chae) Presumably, both i-VNS and t-VNS stimulated myelinated afferent axons in the AÃ range (diameter ≥ 7Âµm) There is no knowledge about the nerve fibre (NF) spectrum of the ABNV, and how it relates to the cervical vagus nerve (CV). Thus, we counted the numbers of all myelinated axons in the ABNV, and compared them with the numbers of presumed afferent axons with diameters ≥7 Âµm in the CV of the same individual.

Materials and methods: 17 bodies donated to the Anatomy Institute I of the University of Erlangen-Nuremberg conventionally embalmed with formaldeyde were used. The ABNV, CV, recurrent laryngeal nerve (RLN) and thoracic vagus nerve (TV) were dissected, postfixed in 2.5% glutaraldehyde in 0.1M phosphate puffer, osmicated and embedded in Epon according to standard protocols. A Leica Aristoplan microscope equipped with an oil-immersion objective (100X, num. aperture 1.32) and a digital camera was used. An average axon diameter was calculated from the long and short axis, including the myelin sheath.

Results: We counted an average of 324 NFs ≥7 Âµm in the TV, 760 in the CV, and 485 in the RLN. Subtraction of 485 (presumed motor axons) from 760 (motor axons and AÃ afferent axons) NFs resulted in an average of 275 AÃ afferent axons in the CV, that roughly corresponded to a direct count of 324 AÃ axons in the TV (containing no motor axons). We counted on average 372 axons in the ABNV (range 180 to 548 ). The largest group of axons was thinly myelinated with diameters between 2 and 5 Âµm. An average of 272 (73% of total, range 139 to 430) of these presumed Aδ fibres were counted. Presumed AÃ fibres measuring ≥7Âµm were much fewer. An average of these 60 axons (22% of all, range 16 to 133) was counted. The group of axons with diameters between 5-7Âµm were not allocated to a defined category but included in the total count. We tentatively considered them as small AÃ axons.

Conclusion: On average, there are five times less AÃ axons in the ABNV which is stimulated in t-VNS than in the CV, the usual site of i-VNS. It remains to be determined if this discrepancy renders t-VNS less effective than i-VNS for treatment.